Chapter 9 Genetics

1. State two laws of heredity that were developed from Mendel’s work.

  1. The passing of traits from parents to offspring is called heredity
  2. From the beginning of recorded history, we have attempted to alter crop plants and domestic animals to give them traits that are more useful to us.

1)Before the discovery of DNA, heredity was a great mystery of science

2)The scientific study of heredity began more than a century ago - Started by Gregor Mendel

  1. He bred pea plants and was able to accurately demonstrate the patterns of heredity
  2. His patterns formed the basis of genetics, which is the branch of biology that focuses on heredity
  3. Note: to “cross” means to mate
  4. He used peas because the garden pea has 2 traits that are clearly different, so they are easy to study
  5. Examples: see the features. Also pea plants are capable of self- pollinating, so they can be easily controlled and they also grow very quickly, so you get a lot of them in a short time - This is useful for obtaining results quickly and having a lot of subjects to count
  6. Useful Features of Peas

a)Flower color – pea flower colors are either purple or white.

b)Seed color – either green or yellow

c)Seed shape – either rough or smooth

d)Pod color – either green or yellow

e)Pod shape – either rough or smooth

f)Flower position – either axial or appendicular

g)Plant height – either short or tall

  1. Mendel Observed that traits are expressed as simple ratios
  2. He used monohybrid crosses
  3. Monohybrid- a cross that involves one pair of contrasting traits
  4. Example: crossing a purple flower with a white flower
  1. How the experiments were done

i.He established a true breeding population (i.e. all the offspring were “pure” for one particular trait).

  1. This is the “Parental Generation” a.k.a. P generation
  1. He then crossed 2 P generation plants that had contrasting forms of a trait (like purple and white flowers)
  2. The resulting flowers were called the first filial generation a.k.a. F1 generation
  3. Finally, he crossed flowers from the F1 generation to form a second filial generation a.k.a. F2 generation
  1. Results:

i.Cross of 2 P1s equals all purple flowers

ii.Cross of 2 F1s equals 3 purple flowers (75%) and 1 white flower (25%)

iii. He found a 3:1 ratio for each of the seven traits he studied

  1. Mendel’s work became the theory of heredity
  2. Prior to Mendel’s work, people just thought offspring were a blend of their parent’s characteristics
  3. Mendel concluded that that each pea has 2 separate “inheritable factors” for each trait, one from mom and one from dad
  4. These factors are now known as genes
  5. They are inherited when 2 gametes fuse

2. Distinguish between the terms Dominant trait, Recessive trait, Genotype, Phenotype, Homozygous, and Heterozygous

  1. The genetic makeup of an organism is its genotype

1)The genotype consists of the alleles that the organism inherits from its parents

  1. For each inherited trait, an individual has 2 copies of the gene
  1. There are alternative versions of genes
  2. Example: purple and white flowers on the pea plants
  3. When 2 different alleles occur together, one may be completely expressed while the other is not observed

i.The expressed allele is called the dominant allele because it masks the effect of the other

  1. Dominant alleles are indicated by writing the first letter of the trait as a capital letter - For example, a purple flower, which is dominant, would be written as P

ii.The unexpressed allele is called the recessive allele because it has no observable affect when paired with a dominant allele – only shows when paired with another recessive copy of the allele

  1. Recessive alleles are indicated by writing the first letter of the dominant trait, but in lower case - For example, a white flower, which is recessive, would be written as p
  1. When gametes form, the alleles for each gene separate independently from one another -Thus, gametes only carry one copy of the allele -When they fuse, the new organism gets 2 copies of the allele
  2. Since there are always 2 alleles, you should have 2 letters to represent your genotype
  1. The appearance of an organism is the result of its genotype - called its phenotype

1)Human phenotypes can appear to be altered by behavior

  1. Hair dye, contact lenses of varying colors, and plastic surgery can all change an individual’s appearance, but they do not alter the individual’s true phenotype or genotype
  1. When both alleles of a pair are alike, the organism is said to be homozygous for that characteristic

1)An organism may be homozygous dominant (written as two capital letters AA) or homozygous recessive (written as two lower case letters aa)

  1. When the two alleles in the pair are different, the organism is heterozygous for that characteristic

1)Represented by one capital letter and one lowercase letter (Aa)

  1. State two laws of heredity that were developed from Mendel’s work.
  1. The Law of Segregation - states that a pair of factors (now known as alleles) is segregated, or separated, during the formation of gametes

1)Alleles for a trait separate when gametes are formed.

2)These allele pairs are then randomly united at fertilization for genetic variation

B.The Law of Independent Assortment states that factors (alleles) for different characteristics are distributed to gametes independently

1)Note: This only applies if the traits are on different chromosomes or if they are far apart on the same chromosome – otherwise they are considered linked and will always be inherited together

4. Use a Punnett square to predict the results of monohybrid and dihybrid genetic crosses.

  1. Punnett Squares can predict the expected outcome in crosses

1)Punnett square - a diagram that predicts the expected outcome of a genetic cross by considering all possible combinations of gametes in the cross

2)Named for Reginald Punnett

  1. The simplest square consists of 4 boxes inside a square - monohybrid

1)The parental gametes are written on the top and sides of the square

2)The letter combinations created indicate the possible genotype of all the offspring

  1. Punnett squares are good for predicting the outcomes of monohybrid and dihybrid crosses -They are also useful for test crossing

1)Test cross - a cross of an individual whose phenotype is dominant but the genotype is unknown

2)This individual is crossed with a homozygous recessive individual

  1. Probabilities can also predict the outcomes of a cross

1)Probability - the likelihood that a specific event will occur

  1. Probability: number of one kind of possible outcome / total number of all possible outcomes
  2. The same formula can be used to predict the probability of an allele being present in a gamete

2)Probability of the outcome of a cross

  1. Since both parents are involved in a genetic cross, both parents must be considered when calculating the probability of the outcomes of a genetic cross
  2. To find the probability that a combination of 2 independent events will occur, multiply separate probabilities of the 2 events - Example: 1/2 x 1/2 = ¼

E.Family Pedigrees can be used to study how traits are inherited

1)Pedigree - a family history that shows how a trait is inherited over several generations

  1. Particularly useful when trying to track genetic disorders
  2. If the trait is autosomal, it will appear in both sexes equally
  3. If a trait is sex-linked, it is usually only seen in males because the trait is located on the X chromosome and boys don't have a second X chromosome to compensate for it
  4. Females will only express the recessive condition if they have both recessive alleles

2)Dominant or Recessive

  1. If the trait is autosomal dominant, every individual with the trait will have a parent with the trait - If the trait is recessive, a person who has the trait may have parents with one, both, or none of the parents exhibiting the trait

3)Heterozygous or Homozygous

  1. Two people who are heterozygous (mixed) carriers of the recessive trait will not show the trait but may pass it on to their offspring
  2. Two people who are homozygous (pure) recessive for a trait always pass it on

4)Patterns of Heredity

  1. Most traits are not controlled by simple Dominant-Recessive alleles
  2. Traits such as hair color, eye color, height, weight, etc. are controlled by multiple genes
  3. When several genes influence a trait, the trait is a polygenic trait
  4. Intermediate traits
  1. A trait that is intermediate between 2 parents exhibits a condition known as incomplete dominance

ii. Example: a red snapdragon flower crossed with a white snapdragon flower produces a pink snapdragon flower

  1. Example 2: a child with wavy hair probably has a parent with straight hair and one with curly hair.
  1. Traits with two forms displayed at the same time are said to be codominant
  1. Codominance- both traits are displayed at the same time

ii.Example: a roan coat in horses means that the horse has both white hair and red hair displayed at the same time

  1. Traits controlled by genes with 3 or more alleles
  1. Genes with 3 or more alleles are said to have multiple alleles
  2. Example: Human blood types A, B, AB, and O
  1. Traits influences by the environment
  1. Some traits can be influence by the conditions in the environment
  2. Example: The artic fox's coat changes colors in response to temperature - In the summer, the fox produces enzymes that make pigments, which gives them a reddish-brown coat - In the winter, these enzymes aren't made anymore because it's cold, so their coats loose their color and turn white - This allows them to blend in with the snow
  3. Example 2: Height in humans is influenced by nutrition
  4. Example 3: Skin color is affected by the sun
  1. Some traits are caused by mutations
  1. The harmful effects produced by inherited mutations are called genetic disorders
  2. Sickle cell anemia - a mutated allele produces a defective form of the protein hemoglobin - This causes the cells to be "sickle" shaped
  3. Hemophilia - a condition in which the blood doesn't clot - This is a sex-linked trait
  4. Huntington's disease (HD) - causes a loss of muscle control, uncontrollable physical spasms, severe mental illness, and eventually death

a)The symptoms don't usually appear until someone is their 30's or 40's so these people don't usually know they have the disease until later in life and generally find out after they have had children

  1. Detecting and treating genetic disorders
  2. Most genetic disorders can't be cured, but there is usually some sort of therapy available to treat the disorders
  3. Genetic screening can be done in-utero to see if there are any problems
  4. Gene therapy is a promising field because someday scientists may be able to correct certain genetic disorders by replacing the defective genes with copies of healthy ones

5. Differentiate a monohybrid cross from a dihybrid cross.

  1. A cross between individuals that involves one pair of contrasting traits is called a monohybrid cross

1)A cross between a pea plant that is pure for producing purple flowers and one that is pure for producing white flowers is an example of a monohybrid cross

  1. A dihybrid cross is a cross between individuals that involves two pairs of contrasting traits

1)A cross involving both seed texture and seed color is an example of a dihybrid cross

Chapter 9 Fundamentals of Genetics

The study of how characteristics are transmitted from parents to offspring is called genetics.

Self-pollination, in which pollen is transferred from the anthers of a flower to either the stigma of the same flower or the stigma of another flower on the same plant, normally occurs in pea plants. Cross-pollination occurs when pollen is transferred between flowers of two different plants.

Mendel concluded that inherited characteristics are controlled by factors that occur in pairs. In his experiments on pea plants, one factor in a pair masked the other. The trait that masked the other was called the dominant trait. The trait that was masked was called a recessive trait.

We now know that the factors that Mendel studied are alleles, or alternative forms of a gene.

The law of segregation states that a pair of factors is segregated, or separated, during the formation of gametes. The law of independent assortment is observed only for genes that are located on separate chromosomes or far apart on the same chromosome.

The law of independent assortment states that factors for different characteristics are distributed to gametes independently. You know this to be true from your study of meiosis: during prophase I of meiosis, homologous chromosomes randomly migrate to opposite sides of the dividing cell.

The genotype is the genetic makeup of an organism. An organism’s phenotype is its appearance as a result of its genotype.

Probability is the likelihood that a specific event will occur. A probability may be expressed as a decimal, a percentage, or a fraction.

A Punnett square can be used to predict the outcome of genetic crosses.

A cross between individuals involving one pair of contrasting traits is a monohybrid cross.

A testcross, in which an individual of unknown genotype is crossed with a homozygous recessive individual, can be used to determine the genotype of an individual whose phenotype is dominant.

Complete dominance occurs when heterozygous individuals and dominant homozygous individuals are indistinguishable in phenotype.

Incomplete dominance occurs when two or more alleles influence the phenotype, resulting in a phenotype intermediate between the dominant trait and the recessive trait.

Codominance occurs when both alleles for a gene are expressed in a heterozygous offspring. Neither allele is dominant or recessive, nor do the alleles blend in the phenotype as they do in incomplete dominance.

A cross between individuals involving two pairs of contrasting traits is called a dihybrid cross.

Vocabulary List

Allele

Codominance

Complete dominance

Cross-pollination

Dihybrid cross

Dominant

F1 generation

F2 generation

Genetics

Genotype

Genotypic ratio

Heredity

Heterozygous

Homozygous

Incomplete dominance

Law of independent assortment

Law of segregation

Molecular genetics

Monohybrid cross

P1 generation

Phenotype

Phenotypic ratio

Pollination

Probability

Punnett square

Pure

Recessive

Self-pollination

Strain

Testcross

Trait